OXYGEN AND CARBON DIOXIDE
Since air contains around 20% oxygen, even the most oxygenated water rarely contains
more than 1% dissolved oxygen. Fish have special organs - branchiae - which allow
them to extract most of this (see Anatomy and Biology, page 42). Oxygen contributes,
in addition, to the respiration not only of plants but also of organisms which are
invisible to the naked eye and often forgotten: the bacteria. The latter transform
the organic matter emitted from living beings (excreta and various other residues),
and these chemical reactions similarly require oxygen.
Simple agitation systems stir the water, enhancing the diffusion of the
oxygen required by fish.
The oxygen in water comes from the dissolution of the oxygen in the air, a process
enhanced by movements in the water produced by wind, currents, or downward flow.
The more water is stirred, the more it is oxygenated. Plants also provide oxygen,
which they produce through photosynthesis, although this process occurs only by
day. The maximum amount of oxygen that water can contain is determined by its temperature:
the higher this is, the less oxygen the water can contain (at 25°C there is 18%
less oxygen than at 15°C).
Oxygen is measured in mg/liter, and its control is quite a complicated matter.
The most turbulent, and therefore the most oxygenated, water contains 8-10 mg/liter,
while the most deficient water sometimes has less than 2 mg/liter.
The oxygen content in an aquarium is usually at its maximum, providing the recommendations
for stirring the water are followed. The rare problems which do occur are the result
of negligence as regards the overall balance of the aquarium (overpopulation of
fish, small number of plants), or non-functioning of equipment due to forgetfulness,
breakdown, or a power cut.
Carbon dioxide derives from the respiration of fish, plants, and bacteria. Stirring
the water enhances its oxygenation, thereby reducing the levels of carbon dioxide
in the water, and passing it into the atmosphere. Carbon dioxide is quite rare in
an aquarium, and this can, to some extent, prove prejudicial to plants, as they
absorb it by day through photosynthesis to extract the carbon they need to grow.
It is therefore vital to establish a permanent equilibrium between oxygen, carbon
dioxide, plants, and fish, although this balance changes at night, when plants stop
Carbon dioxide is also one of the main factors affecting the pH.
Maximum oxygen content of water as a function of temperature
|15 16 17 18 19 20 21 22 23 24 25 26 27 28
||10.1 9.9 9.7 9.5 9.3 9.1 8.9 8.7 8.6 8.4 8.3
8.1 7.9 7.8